Active-RC circuit configurations can include combinations of resistors, capacitors, and operational amplifiers arranged variously to implement continuous time filters and other circuits. FIG. 1 depicts an embodiment of an active-RC continuous-time filter 20. The filter 20 includes a differential amplifier AA, a pair of adjustable resistances RVA, RVB, and a pair of adjustable capacitances CVA, CVB arranged in a differential integrator configuration. The values to which the adjustable resistances RVA, RVB and capacitances CVA, CVB are set can dictate the frequency response of the filter, and thus its suitability for various applications.
FIGS. 2A and 2B depict exemplary embodiments 32, 36 of an adjustable resistance and capacitance, respectively. In FIG. 2A, a plurality of switches SA-SD selectively connect a plurality of resistors RB-RE in parallel with a first resistor RA as a function of a plurality of control signals DA-DD. Turning on selected combinations of the switches SA-SD can decrease the adjustable resistance value from a value equal to that of the first resistance RA, with no additional resistances selected, to a value equal to the parallel combination of all of the resistors RA-RE, with all of the additional resistors selected, or to intermediate values. In FIG. 2B, a plurality of switches SE-SH selectively connect a plurality of capacitors CB-CE in parallel with a first capacitor CA. Turning on selected combinations of the switches SE-SH using a plurality of control signals DE-DH can increase the adjustable capacitance value from a value equal to that of the first capacitor CA, with no additional capacitances selected, to a value equal to the sum of that of all of the capacitors CA-CE, with all of the additional capacitors selected, or to intermediate values. The architecture of FIG. 2B also includes a second plurality of switches SI-SL, driven by inverts DEb-DHb of the plurality of control signals DE-DH, to control the voltage at the floating nodes of the capacitors CB-CE when they are not selected.
When substituting the adjustable resistance and capacitance architectures of FIGS. 2A and 2B into the active-RC integrator 20 of FIG. 1, the node connecting the plurality of impedance selection switches SA-SD, SE-SH, i.e., node A1 in FIG. 2A and node A2 in FIG. 2B, can be connected to summing nodes 24, 28 of the active RC circuit 20, i.e., to the inputs of the operational amplifier AA. This may reduce the signal dependence of the adjustable resistance or capacitance values if the voltage variation at these summing nodes 24, 28 is less than that at the input IN+, IN− or output OUT+, OUT− of the integrator circuit 20, as often may be the case.
However, even if the impedance selection switches SA-SD, SE-SH are connected to the summing nodes 24, 28 of the operational amplifier AA, the adjustable resistance and capacitance architectures of FIGS. 2A and 2B can still suffer from signal-dependent resistance values and output signal distortion. In both FIGS. 2A and 2B, internal nodes of the parallel selectable resistor or capacitor branches, not connected to the summing nodes 24, 28, may still experience signal variations as a function of the on-resistances of the switches SA-SD, SE-SH. These signal-dependent voltages appearing across the switches SA-SD, SE-SH may in turn affect their on-resistance values, and thus the overall adjustable resistance or capacitance values, as a function of the input signal IN+-IN−.
One solution to this problem can include making the on-resistance of the switches SA-SD, SE-SH smaller by making the switches SA-SD, SE-SH wider, thus lessening the voltage division across the switches and the resulting the signal dependence of the adjustable resistance and capacitance values and output signal distortion. However, wider switches can entail greater parasitic capacitances, which can add capacitance to the summing nodes 24, 28 of the integrator 20 of FIG. 1. This can potentially divide the feedback signal delivered to the inputs of the amplifier AA and reduce the feedback factor β by a factor equal to CV/(CV+CP), where CP is the added parasitic capacitance and CV is the nominal feedback capacitance (e.g., CVA, CVB). This reduction in feedback factor may reduce the frequency response, distortion and stability performance of the integrator 20.
Thus, a need exists for switch circuits and switch drivers, for use in active-RC and other circuit configurations, that can be used to provide adjustable impedance values in response to control signals, while exhibiting reduced impedance signal-dependence and reduced circuit performance degradation with respect to frequency response, distortion and stability.